The present invention relates to an electron microscope that is used to measure microscopic dimensions of semiconductor devices, etc., a system thereof, and a method for measuring dimensions using the electron microscope and the system.
In processes of manufacturing semiconductors, a general method employed is to measure dimensions of microscopic patterns by using electron images acquired with an SEM (Scanning Electron Microscope). Further, in general, secondary electron images, not reflected electron images, are used for measurement of the above-stated dimensions. When the term “electron image” is referred to simply as it is, it shall imply a “secondary electron image.”
Now, FIGS. 9A, 9B and 9C show the relationship among a cross-section photo of an object (FIG. 9A), a secondary electron image (FIG. 9B) and a line profile of the secondary electron image (FIG. 9C). Since the secondary electron intensity becomes larger as the slope of an object becomes larger, an image where the portions that correspond to edge areas of the object pattern become brighter and the portions corresponding to flat areas become darker as shown in FIG. 9B. For measurement of dimensions, in some cases, an image is displayed on a GUI window, and the dimension w1 (corresponding to the bottom dimension) and w2 (corresponding to the top dimension) are measured by manually moving the cursor, or alternatively, measurement of dimension is automatically conducted with various edge detection methods as shown in FIGS. 10A, 10B, and 10C. FIG. 10A shows a method for detecting the each point (the each position) of the steepest gradient (the maximum inclination at each slope) as the each edge (the steepest gradient method). FIG. 10B shows a method for detecting the each edge by using a given threshold value (th=max×a+min×(1-a), where, a: a given ratio (0.1 to 1.0)) (the threshold method). FIG. 10C shows a method wherein each straight slope line is fitted to the each edge portion and each straight base line is fitted to the each base material portion, and each intersection of these straight lines is detected as the each edge (the collinear approximation method).
Now, to convert a dimension measured on an image into an actual dimension, the scale of magnification of an SEM should be known in advance. To calibrate the scale of magnification of an SEM, a standard sample whose dimensions are known is used. As stated on page 48 of the “Scanning Electron Microscope”. edited by Kanto Branch of The Japanese Society of Microscopy, the silicon microscale which is created by using anisotropic etching of silicon based on interference patterns generated by using laser beam, for example, is known as the standard sample. Since the silicon microscale carries lines and space patterns arranged in a constant pitch, and the repeated pitch thereof is determined by the wavelength of laser used, it guarantees precise values. The scale of magnification of an SEM can be determined by dividing the pitch distance on an electron image by the pitch distance of the silicon microscale.
Along with miniaturization of semiconductor device patterns, demands on measurement accuracy are becoming stricter year after year. The International Technology Roadmap for Semiconductors (ITRS) demands 0.6 nm of dimension measurement accuracy for 90 nm nodes, and 0.41 nm for 65 nm nodes. Further, the demand includes not only measurement reproducibility of a single SEM measurement apparatus, but also measurement reproducibility among electron microscope apparatuses, or more specifically, matching of measured values when an object is measured with a plurality of electron microscopes. Conventionally, to ensure consistency of measurement values among a plurality of electron microscopes, matching of scales of magnification among electron microscopes have been made by using the above-described standard sample that has known pitches.
In addition, the Japanese Patent Laid-open No. 11-40096 states an electron beam apparatus. In this apparatus, the characteristic best frequency that corresponds to the out-of-focus amount based on the Fourier spectrum of a sample image by using an out-of-focus computing unit. An out-of-focus beam distribution function corresponding to the characteristic best frequency is generated by using a beam distribution generator. The above-stated out-of-focus beam distribution function is removed from the sample image stored in a memory of one-dimensional images by using a deconvolution-computing unit. Measurement of dimensions of the sample image thus obtained is conducted with a length measurement-computing unit.
However, even if the matching of scales of magnification are made among a plurality of electron microscope apparatuses as stated above, the measurement values obtained when an object is measured may not always become matched if the resolution of the electron microscope apparatuses is different. The resolution of an SEM is dependent on the spot diameter (hereinafter referred to as the probe diameter) of the scanning primary electron beam. FIG. 11A shows a line profile of a secondary electron image having a line and space pattern (line pitch: p0, line width: w0) which has a rectangular cross section. FIG. 11B shows the line profile of the secondary electron image in the case where a probe diameter=d1=0. FIG. 11C shows the case where a probe diameter=d1. FIG. 11D shows the case where a probe diameter=d2 (>d1). The line profile based on a finite probe diameter is equivalent to the profile wherein a beam intensity distribution is convoluted on the line profile for the probe diameter 0. Therefore, the profile width which is corresponding to the pattern edge will be expanded as shown in FIGS. 11C and 11D. For example, when edge detection is conducted with the threshold method as shown in FIG. 10B, the dotted line positions on the figure are detected as edge positions.
In the above-described prior art, the matching of scales of magnification is made so that the measurement results p1, p2 and p3 of pattern pitches will conform to actual dimensions. However, as shown in FIG. 10B, the measurement result of the line width becomes larger as the probe diameter becomes larger (w1<w2<w3). As described above, even if the matching of scales of magnification are made, the measurement results of the line width will not agree in general if the probe diameters are different from each other.
The resolution of an SEM is determined by various factors such as, aberrations (spherical aberration and chromatic aberration) of an objective lens, the size of an electron source, and the aperture shape of an objective lens. It is ideal that a plurality of electron microscope apparatuses have the same resolution. However, there are individual differences among electron sources or objective lens apertures, and skills of workers who carry out optical axis alignments vary between individuals. In addition, as time elapses, deterioration of electron sources, or effects of contamination on the objective lens aperture will occur. As a result, it is practically difficult to maintain the status of matched resolution among the electron microscope apparatuses. If a difference in resolution is found, adjustment of an alignment coil (208), adjustment of an astigmatism compensation coil (209), replacement of an objective lens aperture (210), etc., which are shown in FIGS. 1 and 7, will be conducted. However, such adjustments do not always guarantee matched resolution among the electron microscopes.